The invention relates to a shell-and-tube heat exchanger for removing volatile substances from a polymer solution by degassing, which comprises a bundle of parallel vertical tubes arranged within a shell through which a fluid heat transfer medium flows, with the tubes each being fixed at their upper end in an upper tube plate and at their lower end in a lower tube plate and the polymer solution to be degassed flowing through the tubes in the direction of gravity. The invention further provides a method of removing volatile substances from a polymer solution by degassing in a shell-and-tube heat exchanger and also the use of said shell-and-tube heat exchanger.
Thermoplastic polymers such as polystyrene (PS), impact-modified PS (HIPS) or styrene-acrylonitrile copolymers (SAN) are frequently prepared by the solution polymerization process. To obtain the desired polymeric materials, an important process step is the removal of volatile substances, in particular unreacted monomers, low molecular weight reaction products (oligomers), decomposition products, auxiliaries and especially solvents, from the polymer solution obtained after the polymerization.
The isolation of the volatile substances from polymer solutions is frequently carried out by degassing, in which the volatile substances are converted into the gaseous state by introduction of heat and optionally lowering of the pressure and are separated off in this state from the molten polymers.
As regards apparatuses for carrying out degassing, many process variants are known, among which degassing in shell-and-tube heat exchangers has been found to be particularly useful for degassing polymer solutions, in particular in the low- and intermediate-viscosity range. These heat exchangers are apparatuses which have no mechanically moved parts and are relatively unsusceptible to malfunctions.
Shell-and-tube heat exchangers which can be used for degassing comprise, like conventional shell-and-tube apparatuses, a bundle of parallel vertical tubes (i.e. tubes which are arranged parallel to one another and parallel to the direction of gravity) which are fixed at each end in a tube plate. The polymer solution is fed into the tubes and degassed in these; a heat transfer medium is passed through the space between the tubes and heats the polymer solution or melt and also the devolatilization products of the polymer solution. Particularly in the case of large apparatuses having many tubes, it is necessary to distribute the polymer solution to be degassed uniformly over all tubes in order to achieve a uniform product quality.
WO-A 02/00740 describes a shell-and-tube heat exchanger for the degassing of polymer solutions in a first degassing stage, in which a distributor plate (perforated plate) which produces an increased pressure drop in the space before the tubes can be provided in order to distribute the stream entering the shell-and-tube apparatus uniformly over the individual tubes. The process should, in particular, be operated so that no orifice plates or constrictions are provided within the tubes and so that the pressure drop of the flow into the tubes is therefore low. In the process of WO-A 02/00740, a continuous degasser which effects more complete removal of volatile substances is installed in a second degassing stage downstream of the shell-and-tube heat exchanger.
WO-A 07/54504 and WO-A 07/42529 describe shell-and-tube heat exchangers for degassing polymer solutions, which have specific internal fittings within the tubes to produce, inter alia, a preferable step decrease in pressure between tube inlet and degassing region in the interior of the tubes.
When known shell-and-tube heat exchangers and the abovementioned methods of degassing polymer solutions are used, formation of deposits and/or blockages sometimes occur(s) at the entry point of the polymer solution to be degassed into the individual tubes of the shell-and-tube heat exchanger. As a result, cleaning of parts of the shell-and-tube heat exchanger or even replacement of individual tubes and/or any tube inlet nozzles present becomes necessary at regular intervals. This leads to multiday downtimes with correspondingly high production losses and costs. Furthermore, these deposits can become detached and be carried out with the product stream, so that they can block constrictions in the transport path of the product stream and/or remain in the polymer and contaminate this.
It is therefore an object of the present invention to discover shell-and-tube heat exchangers and methods for degassing polymer solutions which, in comparison to known apparatuses and methods, require less frequent shutdowns for cleaning or maintenance and minimize the proportion of visible contamination in the polymer.
We have accordingly found the shell-and-tube heat exchangers mentioned at the outset for removing volatile substances from a polymer solution by degassing, with it being important for the purposes of the invention that a multilayer (i.e. at least two-layer) woven wire mesh whose finest-meshed layer has a mesh opening of from 50 μm to 1000 μm is installed on the upper side of the upper tube plate.
The shell-and-tube heat exchangers, methods and uses according to the invention make it possible to degas polymer solutions with less frequent shutdowns for cleaning or maintenance than is possible using known apparatuses and methods and to produce polymers which have significantly less visible contamination.